Firefox uses the Hunspell spelling checker. The Hunspell spelling
checker uses a pair of .dic and .aff files for its spelling
dictionaries.
You may be able to fix this particular problem by making the
following two fixes to the file "en-US.dic" (US English spelling
dictionary)
1. find the line which says
dean/M
and change it to
dean/MS
2. find the line which says
an/CS
and change it to
an
I have not tested this thoroughly, so make a backup in case you want
to revert.
There does not appear to be any way to specifically add a list of
words to always reject.
--
Kind regards
Ralph